High voltage discharge tube

ABSTRACT

405,176. Discharge apparatus. MULLER AKT.-GES., C. H. F., 24, R÷ntgenstrasse, Hamburg, Germany. June 9, 1933, No. 16575. Convention date, June 23, 1932. [Class 39 (i).] An X-ray or other high voltage tube has a protective casing which comprises a metal cover and which forms a unitary structure with the tube and a rounded solid body is connected to the anode and extends beyond the insulating parts of the structure into the space defined by the metal cover. The body may be connected to the anode by a rod of diameter less than that of the anode, or by a rod of a material having a lower heat conductivity than that of which the anode is made, e.g., of aluminium if the anode is of copper. In one form Fig. 1 a metal casing 1 surrounds the end of an X-ray tube 2 which is enclosed in an insulating jacket 5. The anode 4 is roughened and blackened and is connected by a rod 6 to a rounded body 7. In a modification the anode is mounted on a ring of ferrochromium sealed to the outer glass wall of the tube.

March 3o, 1937. A. BOUWERS HIGH VOLTAGE DISQHARGE TUBE Filed June 12. 1935 /m/emLor ALB ERT BOUWER Patented Mar. 30, 1937 PATENT GFFICE HIGH VOLTAGE DISCHARGE TUBE Albert Bouwers, Eindhoven, Netherlands, as-

signor, by mesne assignments, to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands,

a Dutch company Application June 12, 1933, Serial No. 675,500 In Germany June 23, 1932 4 Claims.

This invention relates to high voltage discharge tubes and more particularly to the cool- 'ing of the electrodes of such tubes. I shall describe my invention in connection with the -5 cooling of the anode of an X-ray tube, although my invention is also applicable to other high voltage devices, for instance, to high power transmitting tubes `or rectifying tubes.

During the operation of X-ray tubes, a large amount of heat isproduced at the anode, and various means have been suggested to dissipate the heat from the anode, so as to prevent its overheating and increase the load-carrying capacity of the tube.

There are various known methods for cooling the anode of an X-ray tube.

vOne known method is by means of a liquid or gaseous cooling agent which for instance, by making the anode partly hollow, can be brought ,-20 close to thefrontalportion of the anode. Means may also be provided to bring about an energetic circulation of the cooling agent and thus improve the heat dissipation.

This method has the drawback that it requires a supply of the cooling agent and the use of separate supply conduits for same or of special high tension leads provided with ducts for the cooling agent, all of which reduces the mobility of the installation. In case of X-ray tubes having grounded protective envelopes for the shockproong of the X-ray tube, the supply of the cooling liquid introduces further diculties. The conduits for the cooling medium have to pass through the protective envelope and these conduits being `also subjected toV a high voltage, require special provisions to prevent a reduction in the effectiveness of the shockproong purported by the envelope.

Another known method of heat dissipation is by means of heat radiation. The heat dissipated by heat radiation is as a rule low, unless the temperature of the anode is kept quit'e high. As it-is Well known, the heat radiation increases With the fourth power of the temperature and to obtain an effective heat dissipation solely by radiation, a very high anode temperature was required. Heat dissipation solely by radiation could be used only in the case of anodes consisting of highly refractory metals, for instance, when using a tungsten plate anode having an operating temperature of 20007 C. and by avoiding as much as possible all heat dissipation through other members. Theheatradiating capacity of the anode can be further increasedA by special provisions, for instance, by blackening the (Cl. Z50-34) surface of the anode. Cooling solely by means of heat radiation, besides requiring a very high anode temperature, is unsuitable except for comparatively small power inputs.

A further method of cooling is by meansof convection. For this purpose a body may be connected to the anode, which body may be provided with cooling ribs and gives oil, to the air passing along it, the heat supplied from the anode.

In the case of tubes for medium voltages and smaller loads convection is used with a fair amount of success, but such an arrangement has drawbacks, in the case of X-ray tubes for heavy loads, in the case of'high voltages and especially in the case of tubes having protective envelopes for their shockproofing.

One of the disadvantages of such cooling means, in the case of higher voltages, is due to the increase oi" the field strength at the edges of the cooling ribs.

Furthermore, in case of tubes for high loads a fan is required to increase the air circulation and when using a protective envelope, even when using a fan, the air enclosed between the surface of the cooling body and between the envelope is liable to become excessively heated, which causes a reduction of its insulating capacity.

In the cooling devices herebefore employed, whether radiation or convection was utilized, the heat dissipation obtained by the radiation and/ or convection, was aimed to match the heat development at the target surface of the anode, or in other words, the cooling system was so organized that the heat developed at the anode was immediately carried outside of the tube at the same rate as it was developed and the maximum instantaneous value of the load which the anode could carry was limited by the ability of the system to carry the heat outside of the tube at such value of the load.

The present invention is based on the realization that during the operation of the tube, Whether such operation be continuous or intermittent, the average value of the power-input is less than is the maximum instantaneous power and that as long as provision is made for sufficient dissipation of the heat developed at the anode to prevent the overheating of the anode and of other members oi the tube for any ccndition of operation of the tube, the heat dissipation by convection, or convection and radiation may be considerably less than is the maximum instantaneous heat developed at the anode.

My invention is of special importance in connection with high voltage X-ray tubes having protective envelopes, in which case the heat accumulator bo dy is surrounded fully or at least in its major part by the metal protective envelope without the interposition of any heat insulating parts, except for the air. The heat from the heat-accumulator is dissipated to the outside of the tube, primarily by convection of the air surrounding the accumulator, without however causing this air, enclosed between the heataccumulator and the protective envelope to assume unduly high temperatures.

My invention obviates as a rule the necessity of using a fan, and even if a fan is used to increase convection as is sometimes the case for tubes of very large input, the fan needs to be only of very small dimensions.

I am aware that it has already been suggested to use in connection with an X-ray tube a plurality of heat absorbing bodies, which could be removably attached to the anode and which were designed to be used in such a manner that after a body attached to the anode has been heated up in operation, it is replaced Vby a second similar but cold body; and when this .second body is heated up, it is replaced by a third body etc.

Such bodies have comparatively small heat capacities and are not heat-accumulators in the sense of the present invention, which requires the accumulator to have a large heat capacity at least of the order of 300 cubic centimeters of water. In addition, to be removable such heat absorbing bodies of the prior art, had to be accessible from the outside and therefore would be unsuited for the tubes which are provided with a pro-tective envelope for their shockproong or for constructions where there is a chance of coming into contact with the absorbing body, which is at the high anode-potential.

In a device according to the invention the heat accumulator is permanently attached t the anode. The heat accumulator gradually heats up during operation and under maximum load condition gradually assumes its final temperature. The heat transfer from the anode to the heat accumulator, the heat capacity of the accumulator, the heat dissipation from the accumulator by convection of the air-and also to some extent by heat radiation-are so correlated that under any operating condition of the tube the anode temperature is kept below a permissible maximum value, whereas the heat accumulator may gradually heat up to a comparatively low final temperature without any of the tube parts or the air within the envelope being overheated.

It is not altogether necessary that all of the heat to be dissipated from the anode, be conducted to the heat accumulator and be dissipated therefrom by convection, as part of the heat may be dissipated directly by the heat radiation of the anode. As a rule, however, I prefer to have the major portion of the heat developed at the anode transferred to the heat accumulator.

With the cooling arrangement according to my invention it is possible to apply a heavier load to a given size X-ray tube and for a longer time than has been possible with previous constructions with which heat conduction or heat radiation was used for the heat dissipation, and at the same time the drawbacks, for instance, reduced mobility, requirement of supervision etc., inherent in devices using special cooling agents, for instance, Water, are avoided.

It should be also noted that in case the heat from the anode is dissipated partly through convection from the heat accumulator and partly through direct heat-radiation from the anode, the proportion between the heat dissipated by convection and by radiation may depend on the operating condition of the tube. For instance, when the tube is used for a short time, high power load, as necessary for diagnostic work, a larger percentage of the heat is dissipated by direct radiation than is the case, for a long duration, relatively small load, as used for iluoroscopic work, this change of proportion being obtained automatically.

The invention will be more clearly understood by referring to the accompanying drawing which represents by way of example several constructions embodying my invention.

Figure 1 is a partly sectionized side View showing the anode portion of an X-ray tube embodying my invention.

, Figure 2 is a partly Vsectionized side View of the anode end of an X-ray tube showing another embodiment of my invention.

Fig. 3 is a partly sectionized side view of the anode portion of an X-ray tube showing another embodiment of my invention.

Referring to Figure 1, in which the cathode end of the tube is not shown-as the device is substantially symmetrical and the cathode may be of any appropriate design-the tube proper consists of two vitreous portions 2-2 which are interconnected by a central metallic cylinder I0, which surrounds the discharge path and the operating ends of the electrodes. The glass walls 2 are surrounded by heat insulating cylinders 5 5.

The metallic cylinder I0 is surrounded by a sleeve I2 serving as an X-ray protective shield. for instance a lead cylinder of suitable thickness, the sleeve I2 being surrounded by a further metal sleeve I4 for example of copper.

The tube is surrounded by a metallic housing I which is connected at the waist of the tube to the sleeve I4. The envelope I flares outwardly toward the two ends of the tube, and is rounded at its two ends. The envelope I is usually con nected to ground as indicated at I5.

Ihe metallic cylinder I 0 is provided with a ray window II and the sleeves I2 and I4 are provided With corresponding openings to permit the X-rays to emerge from the tube.

The cathode structure 3 is supported from one end of the tube (not shown) in the usual manner and the anode 4 is supported from the other end of the tube. As shown, the anode is suppolted from a re-entrant portion I3 of the glass wall 2, for instance, by means of an intermediate ferrochrome ring I6, which is fused to the reentrant portion I3 and to which the anode is suitably secured, for instance, by welding or soldering.

The anode 4 consists of copper or other good heat conductive material and in general conforms to standard constructions, being provided with an obliquely cut-off frontal portion, to direct the X-ray beam through the ray window in a direction perpendicular to the axis of the tube.

`In casefa considerable portion of the heat de veloped at the anode is to be dissipated by direct heat-radiation fromthe anode, I prefer to increase the heat radiating capacity of the anode by mechanically roughening and/or blackening the cylindrical surface of the anode.

The anode is provided with a downwardly extending axial bore into which is inserted a rod member 6 connectinga heat-absorbing body or heat accumulator 1 to the anode. The member 6 is preferably integral with the body 1 and is in permanent and good heat-conducting connection with the anode.

The heat-absorbing body or heat accumulator 1 is shown as having a pear shape, its upper portion being spherical. Preferably the spherical portion of the body 1 and that of the housing I are concentric, which provides for a uniform distribution of the electric field between the body 1 and the envelope- I whereby no points of high field strengths can occur and point-to-point discharges are avoided.

Preferably the radius of the envelope I should be double that of the spherical portion of the body 1, as this relation gives the least electric stress between the body 1 and the housing I, and either reduction or increase of this proportion would cause a less favorable distribution of the electric field. y

i Thebody 1 has a large heat capacity, exceeding that of 300 cubic centimeters of water, and as a rule even greater. For instance,in one type of tube embodying the invention the spherical portion of the body 1 has a radius of 5 cm. and the volume of the heat accumulator is about 500 cubic centimeters. If energy of 500 watts is supplied for four seconds the temperature of such a heat accumulator rises only about 1 C.

In spite of the comparatively large volume of the heat accumulator, it does not cause undue increase in the dimensions of the tube and in the given case the largest diameter` of the protective housing` is only about 20 cm.

An X-ray tube using such a heat-accumulator can be suitably used for comparatively long duration high loads as well as for continuous loads of less value. For instance, a given tube built according to the invention can carry a continuous load of watts, or be continuously operated for 30 minutes at a load of 200 watts; for 18 minutes at a load of 280 Watts, and for 10 minutes at a load of 350 watts. Under any of the above conditions neither the anode nor the heat accumulator, nor the air enclosed within the metallic casing I', nor any of the other parts of the tube will be heated beyond the designated temperature, whereas except for the first condition, the heat developed within the tube isv at any instant in excess of that carried outside of the tube.

While the heat is conducted from the anode to the heat accumulator, the heatfrom the heat accumulator is dissipated mainly through air convection and to a smaller extent through direct heat radiation from the accumulator.

It should` be noted that the amount of heat given off by the heat accumulator needs only to be so large as to obtain a stable condition for a continuous loading of the tube. Thus if the tube in the above example is loaded with 180 watts, after an operation of about 1 hour a stable condition will be established, whereby the heat transferred to the heat accumulator is equal to that dissipated by same.

Onthe other hand in case the load is greater than 180 watts, for instance, 280 watts, the heat transfer to the accumulator `'will be in excess of the heat dissipation by same, and thus the heat accumulators temperature gradually rises to its maximum permissible temperature. As stated, the time required under such condition for the accumulator to reach its maximum rated temperature is about 18 minutes.

With a power input of less than 180 watts the heat accumulator does not reach its maximum operating temperature at all.

For short-duration, continuous or intermittent very high loads, the heat transferred from the anode to the accumulator considerably exceeds the heat dissipation of the-latter. Therefore in many cases it is desirable to take care of such conditions by dissipating part of the heat by direct radiation from the anode.

The relative amount of the anode-heat dissipated by conduction to the body 1 and by direct radiation, depends among other things on the length, cross-section of the rodfmember 6, and of the material used therefor. .1t furthermore depends on the shape and material of the anode and on its surface condition. y

For instance, if it is desired that substantially the entire heat of the anode `be dissipated by transfer to the heat accumulator, the connecting rod 6', as shown in Fig. 3 should be short and o."

alarge diameter and made of a material having good heat conductivity, for instance, of copper. At the same time the anode 4 is given a low heatradiating capacity, for instance, by polishing the anode surface as indicated by I9 in Fig. 3.

On the other hand, if theopposite is required; namely, that the direct heat radiation from the anode should materially contribute to the heat dissipation, at least in case of heavy loads of short duration, the member 6 is made of a material of poorer heat conductivity, for instance of aluminium, and should have greater length and smaller diameter. At the same time the high heat radiating capacity of the anode surface is increased for instance, by roughening and/ or blackening same.

It should be noted that the direct heat radiation of the anode is kept within such limits that it does not unduly heat up either the air surrounding the tube or the protective casing.

It should also be noted that in case of heavy loads of short duration as used in radiological practice, the anode temperature in view of the heavy load, will considerably increase, and as the heat radiation increases with the fourth power,

a larger proportion of the heat will be dissipated by direct radiation from the anode than in the case of small loads, for instance, as used for uoroscopic work, in which latter case the anode temperature remains low and consequently the heat radiation of the anode is small.

Thus while according to my invention the heat capacity of the heat accumulator must be sufficiently large that with heavy loads of short duration the X-ray tube be capable of taking up more heat per second than it can give off to the surroundings of the tube, direct heat radiation of the anode may be used to increase heat dissipation and this heat radiation automatically takes over anv increasing portion of the heat dissipation,

when operating conditions demand it.

If desired the amount o f heat dissipated in case of a heavy loadingvof the tube, can be evenly divided between direct heatl radiation of the anode and heat transfer to the heat accumulator.

CII

For instance, with a tube designed-,to be operated at loads. not exceeding'v 180 watts, the body 'l may be so dimensioned that the amount of heatA given off by convection of` the air surrounding said accumulator is .5 Watt per degree of tem-v perature difference between said bodyY and the protective housing. Thus, forY instance, if the body 'I' is permitted to be heatedup to 170 C. and the casing temperature is 50 C. the amount of heat dissipated by convection is 60 watts. At such temperature the heat accumulator if articially blackened gives oi about 30 watts by direct radiation.

By providing an anode with a roughened and/'orblackened surface, for instance, bypro- Viding a copper anode with a nickel carbide or chromic-oxide surface layer, the anode is given an increased heat radiating capacity. With an anode surface of 150 square centimeters and a substantially uniform anode temperature of 400 C., a radiation of .6 Watt per square cm.can'be obtained so that the anode may give off 90 watts by direct radiation.

To providefor an anode temperaturev of approximately 400 C. and for a temperature of the heat accumulator of less than 200 C., it is re-4 quiredto provide a temperature drop of about 200`C. inthe heat conducting connection betweenv the anode and the heat accumulator by means previously stated.

In case the load on VVthe tube is lessened, the heat dissipationl of the anode will play a less important part.v

In Fig. 2 a construction is shown which is spe- 7 cially well suited for cases where substantially the whole heat dissipation is tobe by heat transfer to the heat accumulator; this construction has also mechanical advantages over'that shown in Fig. 1.

In the construction of Fig. 2 the glass envelope 2 instead of' being provided with a re-entrant end portion, has fusedv to its end a metallic cap 8, preferably of chrome iron, and the anode 4' and heat accumulator 'I' lie directly against the inner and outer faces of the cap, respectively. The connecting rod member 6 of the heat accumulator 1 in this case passes through the cap 8 and is surrounded substantially over its entire length by the anode 4. The anode 4', heat accumulator l and the cap 8 are mechanicallyinterconnected in suitable manner, provision being also made for the hermetic sealing of the evacuated tube.

It should be noted that chrome iron being acomparatively poor heat conductor, the joint between the chrome iron and the glass is' but little affected by the heat transferred from the anode to the vbody v'l".

As in this case the heat resistance of the rod` member 6l is reduced to a minimum, a very efcient heat transfer from the anode to the heataccumulator takes place. -At thesame time such arrangement provides for an exceedingly rugged construction.

It should be well understood that when using a protective casing in an X-ray tube for very high loads, the air circulation, and thus the heat transfer by convection fromthe heat accumulating bodyA can be increased by providing a fan secured to the metal housing; such fan, however, may be quiter small.

Or again, the tube may be cooled down in rest periods by applying some cooling means exterior to the tube, which accelerate the cooling of the heat accumulator.

From-the. foregoing it will` appear that my novel cooling means provide for a heat-accumulator of large heat capacity, permanently secured to they anode, whereby the tube can be subjected to high loads of considerable duration, during whichv loads the heat developed within the tube is inl excess of that which` can be dissipated to the outside and which excess heat is taken up by the heat-accumulator.

Furthermore, if desired a portion of the heat dissipation can take placedirectly by heat radiation from the anode. k

The construction according to the invention provides for X-ray tubes which may be surrounded by a protective casing Without requiring for their cooling a cooling agent and thus Without being subject to all of the limitation required by such arrangement. At the same time my novel cooling means permits the loading of an X-ray tube with greater loads and for longer time periods than has been possible with former constructions utilizing heat radiation and heat convection for the dissipation of the heat.

While I have described my inventionvin connection with specic embodiments and in specic applications, I do not wish `to be limited thereto, but desire lthe appended claims to be construed as Abroadly as permissible in view of the prior What I claim is: i p

1. In combination, an X-ray tube comprising an envelope having a vitreous portion, a cathode, and an anode, an insulating sleeve surrounding said vitreous portion, a protective metal housing surrounding said :sleeve and having a sphericallyshaped end surface whose center lies on the portion 'ofthe longitudinal axis `of said envelope lying outside the vitreous portion and sleeve, and a solidheat accumulating body secured in good heat-transferring relationship to said anode and having a heat capacity at least that of 300 cubic centimeters of Water, said body projecting beyond said vitreous portion and sleeve and having a' spherically-shaped end surface concentric with the `endsurface of the housing and having a radius substantially one-half the radius of said housing surface, saidbody being adapted to radiate at its maximum operating temperature an amount of heat equal to the heat developed in the tube with a load of 180 Watts and said anode having a polished surface and a comparatively high heat capacity to withstand instantaneous loads `greatly' in excess of 180 watts.

2.In combination, an X-ray tube comprising an envelope having a vitreous portion, a cathode, and an anode, a protective metalhousing surrounding saidX-ray tube and having a spherically-'shaped end surface, and a solid heat-accumulating bodyr securedto said tube in good heat-'transferring relationship to said anode and having vaspherically-shaped end surface concentric with the end surface of said housing, the common center point of said surfaces lying on the portion of the tube axis outside the envelope, said body having a heat capacity at least that of 300 cubic centimeters of water and being adapted to radiate an'amount of heat equal to the amount of heat developed in the anode with a loading of 180 watts and said anode having a comparatively high heat capacity to withstand instantaneous loads greatly in excess of 180 Watts.

3. In combination, an X-ray tube comprising an envelope having a vitreous portion, a cathode, and an anode having sufficient heat capacity to withstand instantaneous loads greatly in excess of 180 watts, a protective metal housing for said art.v

envelope, a solid heat-accumulating body having a transverse section of larger area than that of the maximum transverse section of the anode and a heat capacity of at least that of 300 cubic centimeters of water and disposed within and enclosed by said housing coaxially with said envelope and in good heat-transferring relationship to said anode, said body being adapted to radiate heat at its maximum operating rate of radiation when heat is developed in the anode with a continuous loading of 180 watts and having a rounded-end portion projecting beyond said vitreous portion, said housing having a surface arching over the projecting portion of said heat-accumulating body.

4. In combination, an X-ray tube comprising an envelope having a vitreous portion, a cathode, and an anode having a high heat capacity to withstand instantaneous loads greatly in ecess of 180 Watts, a protective metal housing" for said envelope, an insulating sleeve disposed between said vitreous portion and said housing, a solid heat-accumulating body having a transverse section of larger area than that of the maximum transverse section of the anode and a heat capacity of at least that of 300 cubic centimeters of water and disposed within and enclosed by said metal housing coaxially with said envelope and in good heat-transferring relationship to said anode, said body being adaptedfto radiate heat at its maximum operating rate of radiation When heat is developed in the anode with a continuous loading of 180 Watts and having a rounded surface projecting beyond said vitreous portion and insulating sleeve, said housing having a rounded inner surface arching over said heataccumulating body.

ALBERT BOUWERS. 

